Thin film magnetic hard disks have become an integral item in today's ever-present computers to store digital information. A thin film magnetic hard disk typically contains a magnetic recording thin film formed on a supporting substrate. In order to enhance the durability of the hard disk, a protective overcoat of a very hard material, typically an amorphous carbon, is applied, typically by sputtering, over the magnetic recording layer. The amorphous carbon overcoat typically has a diamond-like structure. The overcoat is usually lubricated to further reduce wear of the disk due to contact with the magnetic head assembly. Typically, the lubricant layer is a molecularly thin layer with a thickness less than 50 .ANG., and consisting of 2 to 4 layers of lubricant molecules.
Tribologically speaking, the hard disk surface can be divided into a landing zone, which corresponds to the inner diameter portion of the disc surface, and a data zone, which corresponding to the outer diameter portion of the disc surface. During the stationary stage, the magnetic head is positioned in the landing zone, which also called the CSS (constant start stop) zone. After powered on, the hard disk begins to spin, and the magnetic head will begin to take off from the landing zone. During read/write operations, the magnetic head is caused by the actuator arm to move horizontally to the desired location in the data zone. During power-off, the magnetic head is moved, again by the actuator arm, back to the landing zone. Throughout the entire power on-off operation, the magnetic head takes off and lands, respectively, only once. During each take-off or landing, the magnetic head will glide on the hard disk surface at least for a short distance. Thus, the wear between the magnetic head and the disc surface becomes a very important consideration in the hard disk design. Typically, the hard disk surface is made to have a non-uniform surface roughness, with the landing zone having a greater average roughness, Ra, than that in the data zone. A lubricant layer of appropriate thickness, typically 10 to 50 .ANG. or about 2 to 4 layers of lubricant molecules, is also required to reduce the stiction between the magnetic head and the hard disk surface.
Because stiction is an important consideration affecting the service life of a magnetic disc, it is often tested during production runs of magnetic discs using the so-called CSS (constant start stop) procedure, by which the magnetic head is first placed in the landing zone. Then the hard disk is caused to spin and the magnetic head is caused to take off. Thereafter, the spinning of the hard disk is stopped and the magnetic head is caused to land. The amount of stiction is recorded as a function of time, and the test is repeated until a predetermined amount of wear is detected. The stiction data, including the entire histogram, as well as the average value and standard deviation thereof, can provide important information in the design and manufacturing of hard disks. It can provide important information in the prediction of the service life of a hard disk.
While the magnetic head takes off and lands only once, respectively, during each on-off period of the computer, it constantly flies over the data zone of the hard disk surface during computer power-on. A flying stiction test has been designed to understand the tribological characteristics of the hard disk between the take-off and landing of the magnetic head. In this test, the magnetic head is allowed to fly over the data zone of the hard disk surface for several hours, typically four hours, and the amount of stiction is recorded. This is called flying stiction data.
With conventional thin film heads, the flying stiction over the flying zone is comparable to the sliding stiction in the CSS zone. However, it has been observed that, with the high density magnetic-resistive heads, the measured flying stiction is considerably higher than the sliding stiction. This relatively high flying stiction associated with the use of magnetic-resistive heads is probably attributed to the lower flying height of magnetic-resistive heads. With the conventional thin film heads, the flying height is typically about 2.0.mu. (or about 500 .ANG.). On comparison, the flying height with the magnetic-resistive head is about 1.0.mu. (or about 250 .ANG.). The lower flying height associated with the magnetic-resistive head causes the flying stiction to be substantially increased.
The low flying height of the magnetic-resistive heads also causes another problem. Typically, about 4 .ANG. of the lubricant molecules in the lubricating layer are bonded to the overcoat (i.e., the diamond-like carbon protective overcoat). The rest of them are unbonded, or mobile, molecules. At the low flying height, the magnetic head can pick up, and has shown to have picked up, some lubricant molecules due to van der Waal and/or other intermolecular forces. The lubricant molecules accumulated on the magnetic head will also be dumped onto the loading zone, causing an increase in the sliding friction between the magnetic head and the hard disk surface in the landing zone. This may result in difficulties in landing and take-off of the magnetic head.
A number of patents have taught methods for reducing mobile lubricant molecules by bonding at least a portion of the mobile lubricant molecules to the hard disk surface. U.S. Pat. No. 4,642,246, the content thereof is incorporated herein by reference, discloses a process for covalently bonding a sufficient portion of a functionalized lubricant to the surface of the hard disk by heating.
U.S. Pat. No. 4,960,609, the content thereof is incorporated herein by reference, discloses a process for bonding a fluoroether lubricant to a thin film magnetic recording disk including the step of exposing the lubricated disk to a plasma of an essentially inert gas.
U.S. Pat. No. 5,030,478, the content thereof is incorporated herein by reference, discloses a process for bonding the lubricant to the overcoat which includes the step of irradiating the lubricated disk by UV radiation.
The prior art methods of bonding lubricant molecules to the hard disk also reduce the amount of mobile lubricant molecules in the landing zone. This can increase sliding stiction and cause problems in the taking off and landing of magnetic heads.
U.S. Pat. No. 5,650,900, the content thereof is incorporated herein by reference, discloses a process for magnetic hard disks with zoned lubricant thickness by which a uniform layer of lubricant is first coated over the disk. Then, lubricant is removed from the distinct zones by solvent removal over a periphery of the disc and by removal by laser, so as to form multiple lubricant thickness zones. Thermodynamically speaking, the non-uniform thickness zones cannot be stable, and, over time, the mobile lubricant molecules will migrate from the thick zones to the thin zones and destroy the object of this invention.